Evaluation of the 

Effectiveness of On-Board 

Diagnostic (OBD) Systems in 

Identifying Fuel Vapor Losses 

from Light-Duty Vehicles 

FINAL REPORT 

Version 11 

Contract No. EP-C-06-0-80 

Work Assignment 5-10 

Prepared by: 

Eastern Research Group, Inc. 

Prepared for: 

U.S. Environmental Protection Agency 

Revised by: 

Eastern Research Group, Inc. and 

U.S. Environmental Protection Agency in response to 

External Peer Review comments provided in RTI 

Technical Memo entitled “Peer Review of LDV/ LDT 

OBD and High Evaporative Emission Report, Work 

Assignment 4-01, (RTI 005)”, May 2012 

April 5, 2013
ERG No.: 0218.05.010.001 

Evaluation of the Effectiveness of On-Board Diagnostic (OBD) Systems in 

Identifying Fuel Vapor Losses from Light-Duty Vehicles 

FINAL REPORT 

Version 11 

Contract No. EP-C-06-0-80 

Work Assignment 5-10 

Prepared by: 

Eastern Research Group, Inc. 

3508 Far West Blvd, Suite 210 

Austin, TX 78731 

Prepared for: 

Ms. Connie Hart 

OTAQ Emissions Program Group 

U.S. Environmental Protection Agency 

2000 Traverwood Dr. 

Ann Arbor MI 48105 

April 5, 2013 
i 

Table of Contents 

1.0 INTRODUCTION ............................................................................................................. 1-1

2.0 BACKGROUND .............................................................................................................. 2-1

2.1 OBD Overview ................................................................................................ 2-1

2.2 OBD Monitors and Readiness .......................................................................... 2-2

2.3 OBD DTCs ...................................................................................................... 2-3

2.4 Evaporative Emissions Control System Overview ............................................ 2-4

2.5 Recent Evaporative Emissions Real World Test Programs ............................... 2-5

3.0 ANALYSIS .................................................................................................................... 3-1

3.1 Data Preparation .............................................................................................. 3-2

3.2 Data Analysis Results ...................................................................................... 3-2

3.3 Comparison of SHED Values to Presence of OBD Evaporative System 

DTCs ............................................................................................................... 3-3

4.0 CONCLUSIONS .............................................................................................................. 4-1

List of Tables 

TABLE 1-1. OVERVIEW OF DATASETS USED IN ANALYSIS ......................................................... 1-2

TABLE 3-1. SUMMARY OF MATCHED SHED AND I/M RECORDS USED IN ANALYSIS .................. 3-2

TABLE 3-2. PAIR COUNTS FOR MY96 LIPAN, KEN CARYL, AND DENVER HIGH EVAP 

STUDY PARTICIPANTS COMBINED ................................................................................. 3-3

TABLE 3-3. NUMBER OF PAIRS WITH EVAP DTCS SET VERSUS HIGH AND LOW SHEDS (1.0

G/QHR CUTPOINT)......................................................................................................... 3-4

TABLE 3-4. NUMBER OF PAIRS WITH EVAP DTCS SET VERSUS HIGH AND LOW SHEDS (0.3

G/QHR CUTPOINT)......................................................................................................... 3-4

TABLE 3-5. DETAILS FOR VEHICLES WITH SHED RESULTS > 1 G/QHR (EXCLUDING LIQUID 

AND UNMONITORED VAPOR LEAKS) ............................................................................... 3-6

TABLE 3-6. MODEL YEAR COUNTS FOR HIGH AND LOW SHED PAIRS IN THE DATA .................. 3-9

TABLE 4-1. COMPARISON OF SHED AND OBD RESULTS FOR VEHICLES WITH SHED

EMISSIONS > 1 G/QHR (SUMMARY OF INFORMATION FOR VEHICLES LISTED IN 

TABLE 3-5) .................................................................................................................. 4-2

 
ii 

List of Figures 

Figure 1-1. Electronic HC detector used for MCM Inspections ............................................... 1-2

Figure 1-2. Portable Sealed Housing for Evaporative Determination (PSHED) used in 

Denver High Evaporative Emission Field Studies ........................................................ 1-3
1-1 

1.0 Introduction 

This document is Eastern Research Group’s (ERG’s) final report on evaluating the 

effectiveness of Second Generation On-Board Diagnostic (OBD) systems through a comparative 

analysis of evaporative emissions field data collected in the Denver area during the past four 

years. These field studies were performed through Cooperative Research and Development 

Agreements (CRADAs 471-08 and 471-A-11) between the Assessment and Standards Division 

(ASD) within the Office of Transportation and Air Quality (OTAQ) of the United States 

Environmental Protection Agency (EPA) and the Colorado Department of Public Health and 

Environment (CDPHE), with support provided by ERG under Work Assignments 1-2, 2-2, 2-4, 

3-4 and 4-4 of Contract EP-C-06-080 with the EPA. A wealth of data has been collected at the 

Denver Lipan1 and Ken Caryl2 Inspection and Maintenance (I/M) lanes using a Portable Sealed 

Housing for Evaporative Determination (PSHED) and remote sensing devices. Because the 

studies took place at I/M lanes in Colorado, OBD information is also available for each vehicle. 

CDPHE independently performed additional work during the summer of 2010 in a repair 

effectiveness study conducted at CDPHE’s Aurora and West Tech test centers. During this 

“Denver 2010” study, vehicles were recruited and brought to one of the two CDPHE facilities 

where laboratory-grade Sealed Housing for Evaporative Determination (SHED) testing was 

performed, in addition to other evaporative control system diagnostic tests. Repairs were 

performed on vehicles with elevated evaporative emission levels. Repeat tests were conducted to 

assess the effectiveness of the evaporative emissions repairs performed by CDPHE. 

The focus of this report is on the analysis of evaporative emissions and related data 

collected on model year 1996 and newer light-duty vehicles equipped with OBD systems. 

Results for the Lipan, Ken Caryl and Denver 2010 studies are analyzed in this report, and Table 

1-1 lists the number of 1996 and later OBD equipped vehicles which participated in each of the 

three studies. 

 

1 DeFries, T; Lindner, J; Kishan, S; Palacios, C (2011) “Investigation of Techniques for High Evaporative 

Emissions Vehicle Detection: Denver Summer 2008 Pilot Study at Lipan Street Station” 

2 DeFries, T; Palacios, C; Kishan, S (2012) “Estimated Summer Hot Soak Distributions for the Denver’s Ken Caryl 

IM Station Fleet ” 
1-2 

Table 1-1. Overview of Datasets used in Analysis

Dataset Source Number of 1996+ 

Vehicles 

Data Available in Dataset

Lipan Participant (LP) 24 OBD, MCM*, PSHED**

Ken Caryl Participant (CP) 89 OBD, MCM, PSHED

Denver 2010 Participant (DP) 68 OBD, MCM, PSHED or LSHED 

Total participants (vehicles) in 3 studies 181 OBD, MCM, PSHED or LSHED

*MCM- Modified California Method 

**PSHED – Portable Sealed Housing for Evaporative Determination 

The purpose of this analysis is to evaluate OBD’s ability to identify vehicles with 

elevated evaporative emissions (due to a leak in the fuel/vapor control system). OBD systems 

are not designed for and are generally not capable of determining and setting a code for a leak in 

the fuel system which would result in a liquid leak.3 Three types of data are used in the analysis 

in this report: 

OBD – OBD codes which indicate system or component malfunctions that could result in 

elevated exhaust or evaporative emission levels. During routine I/M inspections, vehicle OBD 

system data (including diagnostic trouble codes (DTCs)) is collected for 1996 and newer 

vehicles. 

MCM – Vehicles in all three studies received a Modified California Method (MCM) 

inspection, which is an under-hood and under-body inspection for evaporative emissions using 

olfactory, visual, and electronic hydrocarbon (HC) detector checks. MCM variables are generally 

discrete. MCM inspections provide several kinds of information including smell/no-smell noted 

by the inspector, visual condition of various fuel system and evaporative emissions control 

system components, and detect/no-detect by the electronic HC detector (shown in Figure 1-1). 

Figure 1-1. Electronic HC detector used for MCM Inspections 

 

3 40 CFR 86.099-17 
1-3 

PSHED and LSHED – Hot-soak evaporative emission estimates were gathered for each 

vehicle in the three studies by placing the vehicle in a Laboratory Sealed Housing for 

Evaporative Determination (LSHED) or a Portable Sealed Housing for Evaporative 

Determination (PSHED). LSHED values are reported as continuous variables with units of 

grams HC/15minutes. In the Lipan and Ken Caryl studies, a PSHED was used, while in the 

Denver 2010 study a PSHED was used for the vehicles tested at the CDPHE’s West Tech 

Center, and a laboratory (conventional) SHED was used for vehicles tested at CDPHE’s Aurora 

laboratory testing facility. Performance of the PSHED has been evaluated in comparison to a 

laboratory SHED in a separate report4 which shows that for fifteen minutes the PSHED had an 

average Recovery of 97.6% and an average Retention of 95.7%, which are within the 

requirements of the laboratory SHED for this time period. Figure 1-2 shows the PSHED used in 

the Denver studies. 

Figure 1-2. Portable Sealed Housing for Evaporative Determination (PSHED) used 

in Denver High Evaporative Emission Field Studies 

 

The analysis in this report is broken out by vehicle model year, OBD evaporative system 

readiness monitor status, and the presence of enhanced evaporative emission controls for 1996-

1998 vehicles. Vehicles with “Not Ready” systems (as described in Section 2.0, Background) 

 

4 Lindner, J; DeFries, T (2012) “Evaluation of Portable SHED Characteristics” 
1-4 

associated with the relevant OBD code categories are delineated in the analysis. The break-outs 

were made to the finest level of detail possible; however, this was dependent on the sample size 

for each category. OTAQ assisted ERG in identifying those 1996-1998 vehicles that were 

equipped with enhanced-evaporative emissions systems, as well as whether a vehicle was 

certified to meet the federal or California emission standards.5

 

5 Note that US EPA enhanced evaporative emission controls for passenger cars and light trucks phased in over the 

1996-1999 model years. 
2-1 

2.0 Background 

2.1 OBD Overview 

Second-Generation Onboard Diagnostics (OBDII, typically referred to simply as OBD) is 

a system used to monitor the status and performance of a vehicle’s powertrain and emissions 

control system components in order to detect malfunctions which could result in an emissions 

increase above a specified threshold. For tailpipe emissions, this threshold is 1.5 times the 

federal certification emission standard.6 For evaporative emissions control systems, the OBD 

system monitors for proper operation of components, adequate purge flow of the evaporative 

control system and vapor leaks equivalent to certain orifice diameters (such as 0.020” or 0.040”, 

depending on which of the applicable OBD standards were followed for certification). All 

model year 1996 and newer light-duty non-diesel on-road vehicles sold in the United States are 

federally required to be equipped with OBD.7 

The OBD system consists of the powertrain control module (PCM), various engine and 

transmission input sensors and components, emission control components, a malfunction 

indicator light (MIL) and a diagnostic link connector (DLC). These systems work together to 

monitor a vehicle’s powertrain and emission control components during vehicle operation and to 

store diagnostic information and alert the driver if a malfunction is identified. Although the 

emission standards to which OBD must comply are federally defined, development of the 

systems and algorithms used to identify malfunctions is the responsibility of each vehicle 

manufacturer. Therefore, monitoring criteria and the amount of time required for a monitor to 

achieve readiness (as defined in the next section) can vary among vehicle manufacturers and 

even vehicle models and model years. In addition, some manufacturers choose to comply with 

more strict standards than federal standards in order to ensure all their vehicles are 50-state legal. 

For example, many manufacturers choose to have their OBD systems certified to detection of a 

0.020” diameter vapor leak required by the California Air Resources Board rather than the less 

strict federal 0.040” requirement in order to maintain model consistency and avoid the 49-state 

certification issue. 

Diagnostic information is stored in the system using a five digit code which provides 

information about the source of the malfunction, and the driver is alerted when the MIL 

(commonly referred to as a check engine light) is illuminated. By connecting an OBD scan tool 

(or an inspection and maintenance analyzer) to the vehicle’s DLC, certain information including 

 

6 Center for Automotive Science & Technology at Weber State University. (2000). On-Board Diagnostics 

Conference 2000 On-Board Diagnostics Manual. (pp. 2-5). Ogden, Utah: Weber State University. 

7 40 CFR 86.094-17 
2-2 

the vehicle’s MIL command status (whether the MIL is commanded on or off) and any stored 

diagnostic trouble codes (DTCs) can be downloaded from the vehicle’s PCM. These DTCs are 

five-digit alpha numeric codes of the format P####. The codes of primary interest in this report 

are the generic (set in federal regulations) P0 series codes (i.e., P0###) defined in SAE J1979, 

since these are common among all vehicle manufacturers and are the codes primarily used in 

vehicle inspection and maintenance programs. 

2.2 OBD Monitors and Readiness 

Before a vehicle’s OBD system can make a determination about whether or not 

malfunctions are occurring, a series of tests are required to evaluate the operational status of the 

various OBD subsystems. These operational tests are categorized into meaningful sets of 

subsystems referred to as monitors. Each monitor has a readiness code that identifies whether or 

not the related subsystem has been evaluated. If a monitor is “ready”, that simply means that 

testing on that particular subsystem has been completed, and results are available (a DTC will be 

stored if a malfunction was identified). On the other hand, if a monitor is “not ready”, testing of 

that particular subsystem has not been completed, so no information is yet available on whether 

or not malfunctions exist in that subsystem. Therefore, a lack of DTCs for a “not ready” monitor 

does not indicate that no malfunctions (i.e., DTCs) are present; it simply means the system 

evaluation is not complete. 

Monitors may be classified as those that are always “ready” (continuous monitors), and 

those that become ready at different times (non-continuous monitors). The three continuous 

monitors include the Comprehensive Component, Misfire and Fuel System monitors, while the 

typical non-continuous monitors are the Oxygen Sensor, Catalyst, Exhaust Gas Recirculation 

(EGR), Evaporative Emissions Control System (evaporative system), Air Injection System 

(AIR), Thermostat and Positive Crankcase Ventilation System (PCV) monitors.8 As the name 

implies, the continuous monitors are always running (these are “critical” monitors which 

primarily protect the catalytic converter). The non-continuous monitors are only evaluated under 

certain powertrain operating conditions (the vehicle’s PCM uses complex algorithms to 

determine when these non-continuous monitors may be run). For this reason, at any point in 

time, a vehicle will have a number of “ready” monitors, and also may have a number of “not 

ready” monitors. Normally, non-continuous monitors which are “not ready” are set to assess 

readiness after each key-on operating event. 

 

8 Birnbaum, R, & Truglia, J (2000). Getting to Know OBDII. asttraining.com, ISBN 0-9706711-0-5, pg, 32. 
2-3 

The amount of time necessary for monitors to achieve readiness varies on a number of 

factors. OBD systems on older model-year vehicles (i.e., model year 1996-2000 vehicles) 

typically take longer to achieve “readiness” than newer vehicles (as monitoring algorithms have 

been refined and improved over the years). In addition, different types of systems require 

different conditions (referred to as enabling criteria) before they can be tested. For example, the 

operating conditions required to test an oxygen sensor are more frequently encountered than the 

operating conditions necessary to evaluate the evaporative system monitor (the enabling criteria 

are met more frequently for an oxygen sensor monitor than for an evaporative system monitor). 

Therefore, after a PCM reset, the oxygen sensor monitor status is usually “ready” before the 

evaporative system monitor status is “ready”, and consequently, more vehicles on the road likely 

have “ready” oxygen sensor monitors than vehicles with “ready” evaporative system monitors. 

Once a monitor’s system evaluation has been completed, the status changes from “not ready” to 

“ready”, and at that time, if any malfunctions are detected, a DTC is stored. However, until the 

monitor is “ready”, no information can be known about whether or not malfunctions are present. 

This assessment cannot be made until the monitor is “ready”. 

2.3 OBD DTCs 

As previously described, once a fault is detected, a 5-digit DTC is stored. This is a 

standardized code which provides information about the type of system or fault, which conforms 

to the following format:9

• Position 1: Device Code: B=Body; C=Chassis; P=Powertrain; U=Network or data link 

• Position 2: Type: 0 = generic (legislated and common among all vehicle manufacturers); 

1 = manufacturer specific 

• Position 3: Specific vehicle system: 

— 1, 2 = Fuel and Air Metering 

— 3 = Ignition System or Misfire 

— 4 = Auxiliary Emission Controls 

— 5 = Vehicle Speed Control and Idle Control System 

— 6 = Computer Output Circuit 

— 7, 8 = Transmission 

• Positions 4 and 5: These last two digits indicate which component or system has a fault 

As an example, P0455 is a powertrain (P) generic (0) code which involves an auxiliary 

emission control system or component (4), and the last two digits indicate this is an “Evaporative 

Emission Control System Leak Detected (gross leak)”10 (this code is typical for a missing, faulty 

 

9 SAE International, J1979 Surface Vehicle Standard, Issued 1991-12, Revised 2004-04, page 33. 

10 Society of Automotive Engineers. (1999, March). On-Board Diagnostics for Light and Medium Duty Vehicles 

Standards Manual, pg. 123. 
2-4 

or incorrectly installed gas cap, among other things). Aftermarket scan tools and I/M analyzers 

can access and download all generic powertrain (P0) codes, and other codes (such as 

manufacturer-specific P1 codes) may be accessible by some scanners and systems, including 

specialized and manufacturer-specific diagnostic equipment. 

Logic for setting a code varies depending on what system or component is being 

monitored. Some codes are immediately set and the MIL is commanded “on” after only one 

occurrence of a fault. However, PCM algorithms may require repeat failures on two successive 

trips before the code is set and the MIL is commanded on. For these algorithms, if a fault is 

identified on the first of two tests, a “pending” DTC is stored. If the fault is again detected on a 

subsequent trip (generally the next consecutive trip), then the “pending” DTC becomes a 

“confirmed” DTC, and the MIL is commanded on. On the other hand, if a fault is not identified 

on the next trip, the “pending” DTC is erased (except for fuel and misfire monitoring codes, 

which aren’t immediately deleted). Since “pending” DTCs indicate that there may be a problem 

but this is not yet confirmed, the MIL is not commanded on for a “pending” DTC. The MIL is 

only commanded on for a confirmed DTC, whether it be a single trip DTC or a two-trip DTC. 

Once a confirmed DTC is stored, a snapshot of engine operating conditions at the time the code 

was set is stored in the PCM’s memory. This snapshot of conditions is called freeze frame data 

and offers clues that can help a repair technician identify and effectively repair the failure. 

As previously described, the PCM can be “reset” (codes, freeze frame data and MIL 

command “on” cleared) by disconnecting the vehicle’s battery or by using the reset feature of a 

scan tool. This also resets all monitors to “not ready”. However, the PCM will also 

automatically erase codes and freeze frame data and turn the MIL off if the fault which triggered 

the DTC is not encountered during three consecutive tests11 (again, excluding fuel and misfire 

monitors, which are tested in a different manner whose description is beyond the scope of this 

general description). 

As a side note, a flashing MIL (as opposed to constant MIL illumination) indicates the 

fault which has been detected poses an immediate threat to the catalytic converter.12 However, 

an immediate threat to the catalytic converter isn’t always represented by a flashing MIL. 

2.4 Evaporative Emissions Control System Overview 

The focus of this report is on OBD monitoring of the evaporative emissions control 

system. The general purpose of the evaporative emissions control system is to prevent release of 

 

11 Birnbaum, R, & Truglia, J (2000). Getting to Know OBDII. asttraining.com, ISBN 0-9706711-0-5, pg, 26. 

12 Birnbaum, R, & Truglia, J (2000). Getting to Know OBDII. asttraining.com, ISBN 0-9706711-0-5, pg, 24. 
2-5 

fuel vapor (raw hydrocarbons) into the atmosphere during vehicle operation, hot soaks and 

diurnal soaks. This is accomplished by a system design which reduces permeation and vapor 

leaks from various materials and connections. Most of the vapor is routed to a carbon canister 

where it is stored and then released into the engine to be burned at certain times during operation 

of the engine. 

Two overall types of evaporative system OBD monitoring exist, non-enhanced and 

enhanced systems. In general, non-enhanced systems only monitor canister purge flow, while 

enhanced systems monitor both purge flow and test for leaks in the vapor recovery system.13 

Consequently, enhanced systems require leak detection sensors and valves which aren’t required 

in non-enhanced systems. In general, hardware for evaporative emissions control systems may 

include vapor transport lines, control solenoids and valves, pumps, pressure sensors, and the 

evaporative carbon canister. 

2.5 Recent Evaporative Emissions Real World Test Programs 

The Coordinating Research Council (CRC) Real World Group has been conducting 

permeation evaporative emissions testing in the E-77 and E-77-2 programs.14 The permeation 

test procedure was refined in the pilot E-77 program before using it in the larger E-77-2 program. 

Most of the vehicles were specifically recruited as aging-enhanced evaporative emission 

vehicles, model years 1996-2000. Three pre-enhanced evaporative emission vehicles and one 

newer near-zero emissions vehicle which met California PZEV evaporative requirements were 

also tested. The focus was on non-ethanol fuel with both 7 and 9 RVP. The program also 

evaluated 24-hour diurnals at the traditional temperature range of 65-105 °F and a higher 

temperature diurnal of 85-120°F. A subset of the vehicles was tested for 72-hour diurnals. A leak 

was implanted in the gas cap of one of the aging enhanced vehicles, and the series of tests were 

run both before and after implanting the leak. The size of the leak was the minimum size 

necessary for detection by OBD, 0.020” effective diameter as previously described, (although 

federal OBD monitoring requirements specify detection of a 0.040” leak, most manufacturers 

certify to California’s 0.020” in order to avoid the 49-state certification issue). The E-77 study 

demonstrated that hydrocarbon emissions can be up to several orders of magnitude larger with an 

 

13 Birnbaum, R, & Truglia, J (2000). Getting to Know OBDII. asttraining.com, ISBN 0-9706711-0-5, pg, 44. 

14 CRC E-77 reports: Haskew, H; Liberty, T (2008) “Vehicle Evaporative Emission Mechanisms: A Pilot Study, 

CRC Project E-77”, Haskew, H; Liberty, T (2010) “Enhanced Evaporative Emission Vehicles (CRC E-77-2)”, 

Haskew, H; Liberty, T (2010) “Evaporative Emissions from In-Use Vehicles: Test Fleet Expansion (CRC E-77-

2b)”, Haskew, H; Liberty, T (2010) “Study to Determine Evaporative Emission Breakdown, Including Permeation 

Effects and Diurnal Emissions Using E20 Fuels on Aging Enhanced Evaporative Emissions Certified Vehicles, CRC 

E-77-2c” 
2-6 

implanted leak. This established the need to define the rate of occurrence of “leakers” in the fleet 

in order to determine the overall impact on the inventory. 

During additional laboratory testing in the E-77-2 and E-77-2c programs, similarly-sized 

leaks were implanted not only in the gas cap, but in other common locations identified during a 

special study conducted in 2008 by CDPHE at the Lipan I/M station (described later in this 

section). The results indicated evaporative emissions leak rates are dependent on location of the 

leak, in particular for vehicles not equipped with On-Board Refueling Vapor Recovery (ORVR). 

Regardless of ORVR, higher leak rates were measured for leaks implanted at the top of the fuel 

tank and at the inlet connection to evaporative control canister as compared to the gas cap. 

The E-77 testing programs confirmed that both liquid and vapor leaks can be a significant 

part of any hydrocarbon inventory. The missing piece of information is the prevalence of these 

leaks in the fleet. A comprehensive program for quantifying these vehicles in the existing fleet 

has not been attempted since the American Petroleum Institute’s “Raw Fuel Leak Survey in I/M 

Lanes” study in 199815 or the California Bureau of Automotive Repair’s “Evaporative Emissions 

Impact of Smog Check” study in 2000.16 The current fleet consists of aging enhanced 

evaporative emissions vehicles which were not part of or were quite new in these earlier studies. 

During the summer of 2008, CDPHE provided access to the Lipan I/M station as well as 

staff and remote sensing equipment for a study to determine the prevalence of vehicles with 

evaporative leaks in the fleet. Certain model year 1981 and newer vehicles were selected (as 

described in Reference 1) and offered participation in evaporative emissions testing, which 

consisted of a 15-minute hot-soak test in a portable SHED (PSHED), a visual, olfactory, and 

electronic HC detector examination of vehicle fuel metering and evaporative emissions control 

system components. Measurements for a comparison of PSHED results with laboratory SHED 

were also made on selected participating vehicles. 

During the following summer of 2009, CDPHE and the Regional Air Quality Council 

(RAQC) performed a follow-on study with EPA reverting to an advisory role. This study was 

performed at the Ken Caryl I/M station in Denver and was developed based on the Lipan 

experience. Vehicles were selected (as described in Reference 2) and offered participation in 

intensive evaporative emissions testing, which consisted of the PSHED hot-soak test, visual, 

olfactory, and electronic HC detector examination of the vehicle. 

 

15 McClement, D. (1998) “Raw Fuel Leak Survey in I/M Lanes,” 

16 Amlin, D.; Carlisle, R.; Kishan, S.; Klausmeier, R.F.; Haskew, H. (2000) “Evaporative Emissions Impact of 

Smog Check,” 
2-7 

Further work was continued in the summer of 2010 by the CDPHE and the RAQC in a 

repair effectiveness study in which vehicles were brought to and tested at CDPHE’s Aurora and 

West Tech laboratories. In these efforts, SHED tests were performed followed by extensive 

diagnostics and repairs. Repeat SHED tests were then performed on vehicles which had received 

repairs. 

Results from these studies suggest vapor leaks can have an impact on the inventory. 

Traditional evaporative emissions testing, as well as the recent E-77 testing, have shown a range 

of evaporative emissions from test vehicles, particularly with aging vehicles. The higher 

emissions levels (but lower than the “gross liquid leakers”) have been associated with slow 

growing vapor leaks which occur as the evaporative emissions control systems age. Vehicles 

with current (Tier 2 and Enhanced) evaporative emissions control technology are now starting to 

age, and information on the prevalence of these higher emissions vehicles in the fleet is still 

being collected. 
3-1 

3.0 Analysis 

This analysis involves the comparison of information on OBD evaporative system DTCs 

with SHED results for the same vehicles. It is not intended to find prevalence rates with OBD 

evaporative codes. The dataset was drawn from three sets of data: the participants in the High 

Evaporative Emission studies from 1) the summer of 2008 at the Lipan I/M testing station in 

Denver, Colorado, 2) the summer of 2009 at the Ken Caryl I/M testing station in Denver, 

Colorado, and 3) the summer and fall of 2010 at the CDPHE Aurora and West Tech laboratories 

in Denver, Colorado. Each of these three datasets include vehicles that were tested for 

evaporative emissions using a PSHED for 15 minutes (at the Aurora station the vehicles were 

tested using an LSHED for 1 hour, but only the first 15 minutes of continuous measurement were 

used in this analysis to compare with the PSHEDs). 

The I/M dataset included 1996 and newer vehicles. Records for vehicles whose I/M 

inspections were aborted or incomplete due to OBD communication problems were removed 

from the dataset. The excluded vehicles are described in more detail in Sections 3.1 (Data 

Preparation) and 3.2 (Data Analysis Results). Each vehicle’s SHED results were then matched 

with the OBD test that was performed immediately before the SHED test. 

In the case of the Lipan and Ken Caryl high evaporative field studies, the vehicles 

received an OBD test as part of their regular I/M inspection prior to solicitation for participation 

in the study. Therefore, the OBD test usually happened several hours or even a day before the 

SHED test occurred. Therefore, because the vehicle was solicited during the I/M inspection 

(immediately before or just after the I/M test was performed, before the motorist left the facility) 

and was then retained by the staff performing the high evaporative emissions study, it was still in 

the same repair state for both the SHED test and the OBD test. 

In the case of the summer 2010 high evaporative emissions study performed at CDPHE’s 

West Tech and Aurora laboratories, vehicles were solicited by letter and then brought to the 

vehicle test station by the owner. The vehicle was kept for several days by the CDPHE staff and 

SHED tested several times, before and after repair. Each time a SHED test was performed, a 

series of six IM240 tests were performed just prior to the SHED test in order to condition the 

vehicle for the SHED testing. The last IM240 with OBD test that was performed of the six prior 

to the SHED test was used to match with the SHED testing for this analysis. Only the before 

repair test for each of the vehicles in this dataset was used for each vehicle to limit the analysis to 

“as received” condition. 
3-2 

The specific number of OBD/SHED data pairs available is described for each of the three 

datasets below. The overall results for the three datasets combined are described in the Data 

Analysis Results section after that. 

3.1 Data Preparation 

Twenty-four PSHED tests were conducted on 24 vehicles during the Lipan High 

Evaporative Emissions Field Study conducted in the summer of 2008. ERG successfully 

matched 17 of these 24 PSHED results with each vehicle’s I/M OBD results (the I/M test was 

either aborted or did not contain OBD data for the other 7 tests - OBD is advisory only in the 

Colorado I/M program, meaning no pass or fail is assigned based on OBDII results). Similarly, 

89 PSHED tests were conducted on 89 vehicles during the Ken Caryl High Evaporative 

Emissions Field Study, and ERG successfully matched 76 of these 89 records with 

corresponding OBD results (as before, the I/M test was either aborted or did not contain OBD 

data for the other 13 tests). 

Eighty-seven PSHED/SHED tests were conducted on 68 vehicles during the Denver 2010 

study (some of the 68 vehicles in the Denver 2010 study received two tests, a “before repair” and 

an “after repair” test). ERG was able to successfully match all 87 of the Denver 2010 study 

PSHED/SHED results with valid OBD results. Table 3-1 provides a summary of the final 

datasets used in this analysis. As shown in this table, in total, ERG successfully matched results 

from 180 PHED/SHED tests with OBD data. Results for comparison of OBD evaporative codes 

with PSHED/SHED results are provided in the following section. 

Table 3-1. Summary of Matched SHED and I/M Records used in Analysis

Study Total # of 

Vehicles 

Total # of 

PSHED/SHED 

Tests 

“Dropped” PSHED/SHED

Tests (those with no 

corresponding OBD record) 

Remaining PSHED/SHED 

Tests matched with OBD 

records 

Lipan 24 24 7 17

Ken Caryl 89 89 13 76

Denver 2010 68 87 0 87

Total 181 200 20 180

3.2 Data Analysis Results 

As shown in the last column of Table 3-1, when the participant vehicle tests from each of 

the three studies were combined, there were a total of 180 PSHED/SHED tests with valid OBD 

matches (for the remainder of this report, both PSHED and SHED results will be generically 

referred to as “SHED” results for clarity). However, some of these SHED tests were performed 
3-3 

after the vehicle was repaired. These after-repair SHED results have been removed from the 

analysis. Also, some of the SHED and OBD test pairs had an evaporative OBD monitor status of 

“Not Ready” at the time of the OBD test, so these test pairs have also been removed from this 

analysis. Table 3-2 summarizes the remaining vehicle SHED and OBD pairs after application of 

each of these filters. 

In this analysis, only test results with “confirmed” evaporative-related DTCs (DTCs in 

the range P0440 to P0469 with a MIL command status of “on”) were considered. No “Pending” 

evaporative DTCs (DTCs in the range P0440 to P0469 but which were “pending” as described in 

Section 2.3, OBD DTCs) were identified in the data used for this analysis. 

Table 3-2. Pair Counts for MY96 Lipan, Ken Caryl, and Denver High Evap Study 

Participants Combined

SHED/OBD Test Pairs

Total matched SHED/OBD test pairs 180

Before repair SHED/OBD test pairs 157

Before repair test pairs with evaporative system monitor status of “Ready” 149

Before repair test pairs with evaporative monitor status of “Ready” and 

with no confirmed DTCs

145

Before repair test pairs with evaporative monitor status of “Ready” and 

with confirmed DTCs and MIL commanded “on” 

4

3.3 Comparison of SHED Values to Presence of OBD Evaporative System DTCs 

To compare the OBD and SHED results for each matched pair of records, each SHED 

result was defined as either high or low using two SHED cut point values; either 0.3 grams of 

total HC emissions per 15 minutes (0.3 g/Qhr), or 1 gram of total HC emissions per 15 minutes 

(1 g/Qhr). Separate analyses were performed by defining “high” SHED test results above each 

of these cut points or below each of these cut points. The SHED cut point of 1 g/Qhr was used 

because it was found to be the approximate average emission level for the first fifteen minutes of 

a laboratory Hot Soak of implanted leaks with 0.02” diameter (the minimum detectable diameter 

of the evaporative OBD systems) in the Coordinating Research Council (CRC) E-77 test 

programs.17 The 0.3 g/Qhr cut point is an estimate of a 15-minute hot-soak portion of the 2 g/hr 

Diurnal + Hot Soak enhanced evaporative emission standard. This value was derived from the 2 

g/hr Diurnal + Hot Soak enhanced evaporative emission standard by assuming 20% of the 2 g/hr 

emissions are due to Hot Soak, and then assuming 75% of that would be in the first 15 minutes. 

 

17 It was found that when leaks were implanted at the gas cap on an ORVR equipped vehicle the emission levels 

were contained. This effect will be system dependent, and it is therefore likely that some designs of ORVR vehicles 

would show uncontrolled levels of emissions for a leak. The average discussed here is approximate for the 

implanted leak locations at the canister connection and at the top of the tank. 
3-4 

The SHED values were compared for pairs with and without confirmed evaporative 

system DTCs. It can be seen from Table 3-2 that only 4 of the 145 records had confirmed 

evaporative DTCs stored. Tables Table 3-3 and Table 3-4 compare the number of paired 

records with confirmed evaporative DTCs and high/low SHED classification for 1 g/Qhr and 0.3 

g/Qhr, respectively. As stated earlier, these tables are provide a comparison of OBD-equipped 

vehicles that were measured with a SHED test, but due to the recruitment methods (see 

references (1) and (2)), the percentages in these tables are not intended to be representative of the 

fleet. The tables also list the percentages of pairs within the four quadrants for each row and 

column. All 4 paired records with confirmed evaporative DTCs had high corresponding SHED 

values (when defining “high” as 0.3 g/Qhr) and 75% had confirmed evaporative DTCs when 

defining “high” as 1.0 g/Qhr. In contrast, 12% of the 145 pairs with no confirmed evaporative 

DTCs had high SHED values (with a 1.0 g/Qhr cut point) and 16% were “high” at the 0.3 g/Qhr 

cut point. In both Table 3-3 and Table 3-4, 85% of the vehicles with high SHED values did not 

have a confirmed evaporative DTC. This could be due to a number of reasons, such as the fact 

that not all high evaporative emissions are due to vapor leaks which the OBD system is designed 

to identify, but the high percentages does indicate additional investigation is warranted. 

Table 3-3. Number of Pairs with Evap DTCs Set Versus High and 

Low SHEDs (1.0 g/Qhr cutpoint)

OBD Evap DTCs not set OBD Evap DTCs set Total

HIGH SHED

(> 1.0 g/15 minutes) 

17 Vehicles

(85% of 20) 

(12% of 145) 

3 Vehicles

(15% of 20) 

(75% of 4) 

20 Vehicles

LOW SHED (<= 

1.0 g/15minutes) 

128 Vehicles

(99% of 129) 

(88% of 145)

1 Vehicles

(1% of 129) 

(25% of 4) 

129 Vehicles

Total 145 Vehicles 4 Vehicles 149 Vehicles

Table 3-4. Number of Pairs with Evap DTCs Set Versus High and 

Low SHEDs (0.3 g/Qhr cutpoint)

OBD Evap DTCs 

not set 

OBD Evap DTCs 

set 

Total

HIGH SHED

(> 0.3 g/15/minutes) 

23 Vehicles

(85% of 27) 

(16% of 145) 

4 Vehicles

(15% of 27) 

(100% of 4) 

27 Vehicles

LOW SHED

(<= 0.3 g/15 minutes) 

122 Vehicles

(100% of 122) 

(84% of 145)

0 Vehicles

(0% of 122) 

(0% of 4) 

122 Vehicles

Total 145 Vehicles 4 Vehicles 149 Vehicles
3-5 

Table 3-5 provides additional vehicle and test information for vehicles with SHED results 

above the 1 g/Qhr designated threshold, including the packet ID, vehicle make, model year, 

model and the evaporative certification family for the vehicle. The “evap_mon_ready” column 

describes the readiness status of the vehicle’s evaporative system monitor. Table 3-5 also lists 

DTCs that were downloaded at the time of the OBD test and indicates whether or not the DTCs 

were categorized as evaporative DTCs (confirmed DTCs in the range of P0440 to P0469). A 

footnote in the table provides definitions of the evaporative DTCs. Additional information on 

these vehicles and all vehicles with PSHEDs > .3 g/Qhr, including non-ready OBD evaporative 

monitor vehicles, is provided in the appendix of this report. 

Table 3-5 also lists comments provided by the vehicle inspectors as they tried to identify 

the source of the vapor leaks using a hand held HC vapor detector. In many instances, the 

vehicle vapor lines were inaccessible. Some were behind sheet metal shrouds and no intrusive 

process was undertaken to get behind these coverings. Based on the inspector comments, the 

make and model year of the vehicle, and any other visual evidence, ERG and OTAQ made a 

determination regarding whether or not a malfunction should be detectable by the vehicle’s OBD 

system. This determination is listed in the column labeled “Should Problem be Detectable by 

OBD?” If the vehicle inspector was not able to identify the source of the leak, then a 

determination was not made regarding whether or not the OBD system should have been able to 

detect the malfunction, and a “?” is listed in the table. The areas of the fuel system inaccessible 

to the HC detector, as mentioned above, were behind shrouds, above the fuel tank, or within 

bundles of hoses packed too tightly to be reached by the HC detector probe. Due to their 

locations and lack of evidence of a liquid leak, leaks located in these inaccessible areas were 

likely to be a vapor leaks, rather than liquid leaks. Liquid leaks would likely occur at connection 

points in the fuel line or the fuel rail and would likely be detectable by the handheld detector. It 

is unlikely these liquid leaks would be detectable by the OBD systems for most vehicles, since 

OBD systems are not designed to detect liquid leaks. 
3-6 

Table 3-5. Details for Vehicles with SHED Results > 1 g/Qhr (excluding liquid and unmonitor

Table 3-5A 

PacketID Evap Family Dataset 

Model 

Year Make Model 

SHED_g/15mi

n PSHED_ov

LIP-22 2FJXR01251BB LIPAN 2002 SUBARU IMPREZA 1.30 YeCRL-540 . CARYL 1998 HONDA ACCORD 2.10 YeLIP-256 XGMXR0124912 LIPAN 1999 OLDS ALERO 2.18 YeCRL-475 YGMXE0111911 CARYL 2000 CHEV ASTRO VAN 2.45 YeHE-6725 WTYXE0095AE1 DENVER 1998 TOYOTA CAMRY 6.30 YeLIP-193 WFMXE0045AAA LIPAN 1998 FORD MUSTANG 11.22 YeHE-6763 VFM1057BYMA DENVER 1997 MERCURY VILLAGER 20.81 YeHE-6011 2FJXR01251CC DENVER 2002 SUBA LEGACY 5.21 YeHE-6799 TFM2045AYPBA DENVER 1996 FORD RANGER 12.21 YeHE-6018 WFMXE0115BAE DENVER 1998 FORD TAURUS 19.93 Ye

Table 3-5B 

PacketID Evap Family Dataset evap_mon_ready DTC codes

Should Malfunction be 

Detectable by OBD? 

Va

LIP-22 2FJXR01251BB LIPAN Yes - Not ascertainable*, ** No CRL-540 . CARYL Yes - Not ascertainable*, ** VapLIP-256 XGMXR0124912 LIPAN No - Not ascertainable*, ** No CRL-475 YGMXE0111911 CARYL No - Yes** VapHE-6725 WTYXE0095AE1 DENVER Yes - Yes*** VapLIP-193 WFMXE0045AAA LIPAN Yes - Not ascertainable*, *** No HE-6763 VFM1057BYMA DENVER Yes - yes - purge solenoid VapHE-6011 2FJXR01251CC DENVER Yes 451|457 Yes*** VapHE-6799 TFM2045AYPBA DENVER Yes 443 Yes-purge solenoid VapHE-6018 WFMXE0115BAE DENVER Yes 443 Yes-purge solenoid Vap

*The source of the high SHED emissions could not be determined from the technician comments in the field studies. **Manufacturer and EPA certification representatives have stated that these manufacturers are certified to California enhanced OBD rediameter orifice detection capability. ***The magnitude of the emissions indicates that this would have been equivalent to at least a 0.040” diameter orifice and therefore shby Federal OBD.
3-7 

Table 3-5C 

PacketID Evap Family Dataset Inspector CommentsLIP-22 2FJXR01251BB LIPAN Nothing FoundCRL-540 . CARYL 

Sniffer leak detected at vapor canister. No non-OEM installations. Did not Canister leak = vapor not liquid. 

LIP-256 XGMXR0124912 LIPAN Nothing FoundCRL-475 YGMXE0111911 CARYL 

Suspect that fuel pump replacement damaged gasket. Areas on top of tank pipe connect set sniffer off. Also evap canister set sniffer off. No access to of fuel system. 

HE-6725 WTYXE0095AE1 DENVER Gas cap was cause of leak.LIP-193 WFMXE0045AAA LIPAN Nothing FoundHE-6763 VFM1057BYMA DENVER Vapor/purge hose cracked.HE-6011 2FJXR01251CC DENVER Vapor lines at the top of filler neck not connected.HE-6799 TFM2045AYPBA DENVER

Found wires corroded/broken that activated the purge valve. Supplying poshowed the purge valve did not operate. Removed canister and evacuated 

HE-6018 WFMXE0115BAE DENVER Extreme HC odor at right rear. Exhaust collector is right behind this area.

Table 3-5 Legend • P0440 Evaporative Emission Control System Malfunction • P0441 Evaporative Emission Control System Incorrect Purge Flow • P0442 Evaporative Emission Control System Leak Detected (small leak) • P0443 Evaporative Emission Control System Purge Control Valve Circuit • P0444 Evaporative Emission Control System Purge Control Valve Circuit Open • P0445 Evaporative Emission Control System Purge Control Valve Circuit Shorted • P0446 Evaporative Emission Control System Vent Control Circuit Malfunction • P0447 Evaporative Emission Control System Vent Control Circuit Open • P0448 Evaporative Emission Control System Vent Control Circuit Shorted • P0449 Evaporative Emission Control System Vent Valve/Solenoid Circuit Malfunction 

• P0451 Evaporative Emission Control System Pre

Range/Performance • P0452 Evaporative Emission Control System Pre• P0453 Evaporative Emission Control System Pre• P0454 Evaporative Emission Control System Pre• P0455 Evaporative Emission Control System Lea• P0456 Evaporative Emissions System Small Leak • P0457 Evaporative Emission Control System Lea• P0458 Evaporative Emission System Purge Contr• P0459 Evaporative Emission System Purge Contr• P0465 Purge Flow Sensor Circuit Malfunction • P0466 Purge Flow Sensor Circuit Range/Perform• P0467 Purge Flow Sensor Circuit Low Input • P0468 Purge Flow Sensor Circuit High Input • P0469 Purge Flow Sensor Circuit Intermittent

• P0450 Evaporative Emission Control System Pressure Sensor Malfunction 
3-8 

As shown in Table 3-3, twenty vehicles with an evaporative system monitor status of 

“ready” had SHED measurements greater than 1 g/Qhr. Details of these 20 vehicles are listed in 

Appendix A. However, it can be seen that only 10 vehicles with SHED measurements greater 

than 1 g/Qhr are listed in Table 3-5, and of these only 8 have evaporative monitors with a 

“ready” status (as opposed to all 20 vehicles in Table 3-3). The following reasons explain 

differences between the “ready” vehicles emitting greater than 1 g/Qhr in Table 3-5 and the 

vehicles listed in Table 3-3: 

• Table 3-5 includes two vehicles (vehicles associated with packet IDs LIP-256 and 

CRL-475) whose evaporative system monitor status was “not ready” (these are 

not included in the counts provided in Table 3-3). Appendix A provides details of 

vehicles with both “ready” and “not ready” evaporative system monitors. 

• OBD evaporative system monitoring is not designed to identify liquid leaks; 

since Table 3-5 was developed to assess OBD’s ability to identify monitored 

vapor leaks, these vehicles with (unmonitored) liquid leaks which are included in 

Table 3-3 are not listed in Table 3-5. 

 

• Since many 1996 and 1997 vehicles which conform to Federal (non-California) 

standards were only designed to monitor purge solenoid checks but not vapor 

leaks18, those model year 1996 and 1997 vehicles included in Table 3-3 for which 

the inspection indicated the malfunction was anything other than a purge solenoid 

were excluded from Table 3-5, as these would be malfunctions not monitored by 

OBD. Again, details on each of these vehicles are provided in Appendix A. 

• As OBD requirements developed and phased in, it became common practice for 

auto companies, such as those listed in Table 3-5, to certify to the California OBD 

leak detection standard of 0.020” all of their vehicles sold federally. Laboratory 

data has shown a 0.020” diameter orifice to be roughly equivalent to an emission 

rate of 1 g/Qhr. Using the formula for the area of a circle (πr

2

), it can be seen that 

the magnitude of emissions quadruples with the doubling of the diameter size to 

0.040”, and this has been found to be the case with laboratory testing comparing 

0.020” and 0.040” leaks. Consequently, based on the magnitude of emissions, for 

the vehicles in Table 3-5 with SHED emissions of 6.30 g/Qhr and greater, it is 

reasonable to assume that they have leaks of diameter 0.040”. 

Based on these two datasets, this preliminary analysis suggests that OBD systems did not 

identify 50-70% of the potential high evaporative emitters in these studies. The lower range, 

50% (3 out of 6), is based on the occurrences of when the OBD system should probably have 

detected the leak from a known source based on the technician comments, disregarding the “?” 

vehicles in the “Should Malfunction be Detectable by OBD?” column. The higher range, 70% (7 

 

18 40 CFR 86.094-17 
3-9 

out of 10), includes the unknown leak sources (“?”) for which no leak was identified or the vapor 

leak was likely located in the area of the evaporative control system inaccessible to the HC 

detector. Liquid leaks (which would generally not be monitored by OBD) were not included in 

this estimate. 

It should be noted that this preliminary analysis was based on a qualitative review of the 

technician comments, rather than a quantitative assessment of OBD monitoring criteria (such as 

a pressure drop associated with a 0.020” or 0.040” diameter orifice leak). In addition, since 2 of 

these 10 vehicles had a “not ready” evaporative system monitor, it is possible that the vehicle’s 

OBD system would have detected the leak and stored a DTC once the evaporative system 

monitor achieved readiness. However, as most I/M programs in the United States typically allow 

1 to 2 “not ready” monitors, these vehicles could potentially pass an I/M OBD inspection with 

these types of evaporative system failures before the monitor achieved readiness. 

Some of the vehicles in these studies were model year 1996 through 1998 vehicles and 

therefore may not have been equipped with OBD enhanced evaporative system monitoring. For 

these older vehicles, some of the evaporative system failures could be undetectable by the OBD 

systems installed on those vehicles. ERG compared the model year distributions with high/low 

SHED categories to determine if a correlation existed between SHED values and vehicle model 

year. The high SHED pairs included vehicles with model years from 1996-2003, and the low 

SHED pairs included vehicles with model years from 1996-2010. This suggests that high SHED 

results tended to be associated with older model year vehicles. Table 3-6 lists model year counts 

for high and low SHED pairs vehicles of model year 1996-2010 in the dataset. 

Table 3-6. Model year Counts for High and Low SHED Pairs in the Data

Model 

Year

SHED Cut Point of 1.0 g/Qhr SHED Cut Point of 0.3 g/Qhr

# of Pairs with 

SHED Value High 

# of Pairs with 

SHED Value Low 

# of Pairs with 

SHED Value High 

# of Pairs with 

SHED Value Low 

1996 5 9 7 7

1997 7 13 9 11

1998 4 10 4 10

1999 - 10 1 9

2000 2 12 2 12

2001 - 15 1 14

2002 2 17 2 17

2003 - 12 1 11

2004 - 9 - 9

2005 - 13 - 13

2006 - - - -

2007 - 1 - 1
3-10 

Table 3-6. Model year Counts for High and Low SHED Pairs in the Data

Model 

Year

SHED Cut Point of 1.0 g/Qhr SHED Cut Point of 0.3 g/Qhr

# of Pairs with 

SHED Value High 

# of Pairs with 

SHED Value Low 

# of Pairs with 

SHED Value High 

# of Pairs with 

SHED Value Low 

2008 - 2 - 2

2009 - 4 - 4

2010 - 2 - 2

Total 

Vehicles 

20 129 27 122
4-1 

4.0 Conclusions 

Three different datasets were used in this study to determine if high evaporative emitters, 

as defined by LSHED or PSHED tests, were appropriately identified by each vehicle’s OBD 

system. Although the total number of observations was limited, this analysis indicates that many 

vehicles with high SHED values (hot-soak emissions equal to or greater than 1 g/Qhr as 

measured by a SHED test) do not have stored evaporative DTCs or MILs. For vehicles with 

high SHED HC measurement results, an attempt was made to determine the source of the vapor 

leak. Results are shown in Table 4-1, which summarizes pertinent information for all vehicles 

listed in Table 3-5. Due to the configuration of the vehicle chassis the source or sources of each 

leak could not always be isolated. Table 4-1 shows that OBD systems did not identify 50-70% of 

the high evaporative emitters in these studies (for various reasons including “not ready” 

evaporative system monitors). The lower range, 50% (3 out of 6), is based on evaporative 

system failures that should probably have been detected by the OBD system. The higher range, 

70% (7 out of 10), includes vehicles for which no leak source was identified or the vapor leak 

was likely located in an area of the evaporative control system inaccessible to the HC detector 

(“Not ascertained” in Table 4-1). Liquid leaks (which generally are not monitored by OBD) 

were not included in this estimate. 

From an overall perspective, this study identified 29 vehicles (27 listed in Table 3-4 plus 

2 with “not ready” evaporative monitors) with PSHED values greater than 0.3 g/Qhr (roughly 

equivalent to the hot-soak component of the 2 g/hr Diurnal + Hot Soak standard), and only 4 of 

those 29 vehicles had stored evaporative DTCs. Two vehicles had “not ready” evaporative 

monitors. The source of elevated evaporative emissions from the remaining 23 “ready” vehicles 

which had no evaporative DTCs was either undetectable by current OBD design (for example, a 

hidden liquid leak not observed by field technicians), was from a part of the system not 

monitored by OBD, or was a vapor leak which should have been identified but was not (i.e., the 

evaporative emissions monitoring system was malfunctioning in some way). 

No “OBD false positive” vehicles were identified in this study (those with stored 

evaporative system DTCs with SHED emissions under 0.3 g/Qhr). The preferential recruiting of 

vehicles with high evaporative emissions would tend to reduce the likelihood of this category of 

vehicles from participation in these studies, so no conclusions can be drawn from the lack of 

these vehicles in these results. However, the key finding of this report is the high number of 

vehicles with high evaporative emissions which were not identified by the OBD evaporative 

system. 
4-2 

Since the sample size used for this analysis was limited to ten vehicles which were 

expected to set DTCs, EPA sought to enhance our understanding with larger OBD datasets. 

Analysis was performed using inspection and maintenance data from Texas and California. 

Preliminary conclusions from detailed analysis of data from these states complement the 

conclusions of this report and can be found in Appendix B. 

Table 4-1. Comparison of SHED and OBD Results for Vehicles with SHED 

Emissions > 1 g/Qhr (Summary of Information for Vehicles Listed in Table 3-5) 

Data Set Model

Year 

SHED 

g/15 

min 

Evap 

DTC 

Evap 

Monitor 

Ready 

Evaporative 

DTC code 

Should Malfunction 

be detectable by 

OBD? 

Lipan 2002 1.30 No Yes - Not ascertainable*, ** 

Ken Caryl 1998 2.10 No Yes - Not ascertainable*, ** 

Lipan 1999 2.18 No No - Not ascertainable*, ** 

Ken Caryl 2000 2.45 No No - Yes** 

Denver 1998 6.30 No Yes - Yes*** 

Lipan 1998 11.22 No Yes - Not ascertainable*, *** 

Denver 1997 20.81 No Yes - Yes – purge solenoid 

Denver 2002 5.21 Yes Yes 451/457 Yes*** 

Denver 1996 12.21 Yes Yes 443 Yes – purge solenoid 

Denver 1998 19.93 Yes Yes 443 Yes – purge solenoid 

*The source of the high SHED emissions could not be determined from the technician comments in the field studies. 

**Manufacuter and EPA certification representatives have stated that these manufacturers certified to California 

enhanced OBD requirement of a 0.020” diameter orifice detection capability. 

*** the magnitude of the emissions indicates that this would have been equivalent to at least a 0.040” diameter 

orifice and therefore should have been detected by OBD on a federally certified vehicle 
Appendix A 
A-1 

Appendix A: Details for All Study Vehicles with SHED Results > 0.3 g/Qhr 

PacketID Evap Family Dataset Model Year Make Model 

SHED 

g/15min 

PSHED 

> 1 

PSHED 

> .3 

evap 

DTC 

LIP-254 TCR1073AYPBP LIPAN 1996 JEEP CHEROKEE 0.33 No Yes No CRL-568 VGM1035AYPAA CARYL 1997 STRN SL 0.39 No Yes No CRL-393 . CARYL 2001 TOYOTA SOLARA 0.46 No Yes No CRL-546 TNT1047DYMAO CARYL 1996 GEO PRIZM 0.62 No Yes No HE-6454 XFMXE0120BAE DENVER 1999 MERCURY MOUNTAINEER 0.67 No Yes No CRL-395 VGM1095AYMEA CARYL 1997 CHEV S10 0.72 No Yes No CRL-271 VGM1098AYMBA CARYL 1997 GMC SIERRA 1.28 Yes Yes No LIP-22 2FJXR01251BB LIPAN 2002 SUBA IMPREZA 1.30 Yes Yes No HE-6418 VFM1065BYMAA DENVER 1997 FORD ASPIRE 1.47 Yes Yes No CRL-540 . CARYL 1998 HOND ACCORD 2.10 Yes Yes No LIP-256 XGMXR0124912 LIPAN 1999 OLDS ALERO 2.18 Yes Yes No CRL-357 VNS1110AYMEA CARYL 1997 NISS PATHFINDER 2.32 Yes Yes No CRL-475 YGMXE0111911 CARYL 2000 CHEV ASTRO VAN 2.45 Yes Yes No HE-6331 TFM1045AYPAB DENVER 1996 FORD CONTOUR 2.79 Yes Yes No HE-6609 YGMXE0095905 DENVER 2000 OLDS BRAVADA 5.77 Yes Yes No CRL-85 TCR1073AYPOB CARYL 1996 DODG RAM 1500 6.08 Yes Yes No HE-6725 WTYXE0095AE1 DENVER 1998 TOYOTA CAMRY 6.30 Yes Yes No HE-6647 YSZXT0095ME0 DENVER 2000 ISUZU RODEO 8.65 Yes Yes No HE-6759 VGM1070AYMAA DENVER 1997 OLDS DELTA 88 9.10 Yes Yes No LIP-193 WFMXE0045AAA LIPAN 1998 FORD MUSTANG 11.22 Yes Yes No CRL-321 VGM1095AYMEA CARYL 1997 CHEV S10 12.33 Yes Yes No HE-6620 TNS1057BYMBB DENVER 1996 NISS QUEST 12.79 Yes Yes No HE-6519 YTY1047DYMA0 DENVER 1997 TOYT COROLLA 16.41 Yes Yes No HE-6702 TGM1089AYMEA DENVER 1996 CHEV CAVALIER 19.53 Yes Yes No HE-6763 VFM1057BYMA DENVER 1997 MERCURY VILLAGER 20.81 Yes Yes No CRL-316 3HYXR0105PEA CARYL 2003 HYUN ACCENT 0.69 No Yes Yes HE-6011 2FJXR01251CC DENVER 2002 SUBA LEGACY 5.21 Yes Yes Yes HE-6799 TFM2045AYPBA DENVER 1996 FORD RANGER 12.21 Yes Yes Yes HE-6018 WFMXE0115BAE DENVER 1998 FORD TAURUS 19.93 Yes Yes Yes 
A-2 

Appendix A: Details for All Study Vehicles with SHED Results > 0.3 g/Qhr 

PacketID Evap Family Dataset Model Year Make Model 

Should 

Malfunction 

be 

detectable 

by OBD? 

Vapor/ 

Liquid/ 

No 

Leak 

Found 

LIP-254 TCR1073AYPBP LIPAN 1996 JEEP CHEROKEE 

no evap codes yet* Vapor 

Gas cap IM Nothing fo

CRL-568 VGM1035AYPAA CARYL 1997 STRN SL 

no evap codes yet* Vapor 

Gas cap IM Sniffer goof fuel tanOdor of gatank area. 

CRL-393 . CARYL 2001 TOYOTA SOLARA ? Vapor 

Underbodaccessible pretty mucmanifold. 

CRL-546 TNT1047DYMAO CARYL 1996 GEO PRIZM 

no evap codes yet* Vapor 

While lyinthought I gBruce said PSHED do

HE-6454 XFMXE0120BAE DENVER 1999 MERCURY MOUNTAINEER ? 

No Leak Found No comm

CRL-395 VGM1095AYMEA CARYL 1997 CHEV S10 

no evap codes yet* Vapor 

Tank is seliquid fuel activates swhining w

CRL-271 VGM1098AYMBA CARYL 1997 GMC SIERRA 

no evap codes yet* Vapor 

Cannot seFuel tank Around to(right) the near the fr

LIP-22 2FJXR01251BB LIPAN 2002 SUBA IMPREZA ? 

No Leak Found Nothing F
A-3 

Appendix A: Details for All Study Vehicles with SHED Results > 0.3 g/Qhr 

PacketID Evap Family Dataset Model Year Make Model 

Should 

Malfunction 

be 

detectable 

by OBD? 

Vapor/ 

Liquid/ 

No 

Leak 

Found 

HE-6418 VFM1065BYMAA DENVER 1997 FORD ASPIRE 

no evap codes yet* Vapor 

No visible HC from gbroken. Futhe mount down the during repup during 

CRL-540 . CARYL 1998 HOND ACCORD ? Vapor 

Sniffer leaNo non-Ofind fuel fnot liquid. 

LIP-256 XGMXR0124912 LIPAN 1999 OLDS ALERO ? 

No Leak Found Nothing F

CRL-357 VNS1110AYMEA CARYL 1997 NISS PATHFINDER 

no evap codes yet* Vapor 

There is sosets sniffeand canist

CRL-475 YGMXE0111911 CARYL 2000 CHEV ASTRO VAN yes Vapor 

Suspect thdamaged gand near tsniffer offsniffer offthat part o

HE-6331 TFM1045AYPAB DENVER 1996 FORD CONTOUR 

no evap codes yet* Vapor Significan

HE-6609 YGMXE0095905 DENVER 2000 OLDS BRAVADA no Liquid Visible fuCRL-85 TCR1073AYPOB CARYL 1996 DODG RAM 1500 

no evap codes yet* Liquid 

Tank lookgasoline. I lying undeThe vent llooks like inconsistethat area. 
A-4 

Appendix A: Details for All Study Vehicles with SHED Results > 0.3 g/Qhr 

PacketID Evap Family Dataset Model Year Make Model 

Should 

Malfunction 

be 

detectable 

by OBD? 

Vapor/ 

Liquid/ 

No 

Leak 

Found 

HE-6725 WTYXE0095AE1 DENVER 1998 TOYOTA CAMRY yes Vapor Gas cap wHE-6647 YSZXT0095ME0 DENVER 2000 ISUZU RODEO no Vapor 

There is a Sniffer in intake man

HE-6759 VGM1070AYMAA DENVER 1997 OLDS DELTA 88 

no evap codes yet* 

No Leak Found No comm

LIP-193 WFMXE0045AAA LIPAN 1998 FORD MUSTANG ? 

No Leak Found Nothing F

CRL-321 VGM1095AYMEA CARYL 1997 CHEV S10 

no evap codes yet* Liquid 

Major gason engineexactly. Soinjector to 

HE-6620 TNS1057BYMBB DENVER 1996 NISS QUEST 

no evap codes yet* Vapor 

Major HC side. 

HE-6519 YTY1047DYMA0 DENVER 1997 TOYT COROLLA 

no evap codes yet* 

No Leak Found 

No commComment trouble coflow. Invenot in IM. 

HE-6702 TGM1089AYMEA DENVER 1996 CHEV CAVALIER 

no evap codes yet* Liquid 

#3 fuel injmissing, #leaking. 

HE-6763 VFM1057BYMA DENVER 1997 MERCURY VILLAGER 

yes - purge solenoid Vapor Vapor/pur

CRL-316 3HYXR0105PEA CARYL 2003 HYUN ACCENT 

small leak - .02", fuel level sensor Vapor 

Something leaking at active mantank joint. have evapbut that di
A-5 

Appendix A: Details for All Study Vehicles with SHED Results > 0.3 g/Qhr 

PacketID Evap Family Dataset Model Year Make Model 

Should 

Malfunction 

be 

detectable 

by OBD? 

Vapor/ 

Liquid/ 

No 

Leak 

Found 

HE-6011 2FJXR01251CC DENVER 2002 SUBA LEGACY Vapor 

Vapor lineconnected

HE-6799 TFM2045AYPBA DENVER 1996 FORD RANGER 

purge solenoid Vapor 

Found wiractivated tpower and valve did ncanister an

HE-6018 WFMXE0115BAE DENVER 1998 FORD TAURUS 

purge solenoid Vapor 

Extreme HExhaust coarea. 

* 1996 and 1997 vehicles and 1998 federal-only OBD vehicles did not have OBD evaporative system leak checks but did have purge so
Appendix B